Why Full Coverage Matters: Directional Troweling and Back-Buttering

The short answer: waterproofing can be done properly and the shower can still mold, rot and smell. If the installer doesn’t get full thinset coverage under the tile and leaves hollow spots, water finds those pockets and sits there. The membrane is only as good as the coverage under it.

The membrane needs a solid bed

A good shower starts with a membrane. We use LATICRETE HYDRO BAN, two coats, flood-tested before any tile (more in how we waterproof a shower). But the membrane is only one layer. The tile on top has to be bonded solid to it.

When tile is set on blobs of mortar instead of a full bed, there are empty pockets behind it. Those hollow spots can hold water. The tile loosens, grout and silicone crack, more water gets in, and the moisture has nowhere to go. That’s how a shower with a perfectly good membrane still ends up with mold, rot and a smell you can’t scrub out.

Watch out: a good membrane under a bad tile job. Waterproofing doesn’t fix hollow tile. If the coverage isn’t there, the shower can still fail. Best way? Do your research and hire a pro.

What full coverage looks like

Showers call for about 95% mortar coverage, with edges and corners supported. In practice that means:

• A notch trowel sized for the tile.
• Ridges combed in one direction.
• A thin, flat coat back-buttered onto the tile.
• Pressing and sliding the tile across the ridges to collapse them.
• Pulling a tile now and then to check.

We cover the same method for big tile in setting large tile without lippage.

Why directional troweling matters

Directional troweling means combing the thinset ridges in one direction, in straight lines, not swirls or circles. When the tile is pressed and slid across those straight ridges, they collapse into a solid bed and the air has a clear path out. Swirled or random ridges can trap pockets of air behind the tile, and those pockets become the same hollow spots this page warns about.

Directional troweling and back-buttering work together: the combed ridges on the wall or floor, a thin flat coat on the back of the tile, then press and slide. It’s the method in setting large tile without lippage and in how we build a shower step by step.

Watch out: swirled thinset. Ridges combed in circles can trap air behind the tile. Straight ridges in one direction, tile slid across them. Best way? Do your research and hire a pro.

Why back-buttering matters

Back-buttering is a thin, flat coat of thinset on the back of the tile, keyed in with the flat side of the trowel before the tile goes on the wall or floor. Pressing it in keys the mortar into the back of the tile. It fills the low spots and texture on the back of the tile, so the combed ridges have something to bond into. Skip it and you can get gaps even with a good trowel job, especially on large tile. LATICRETE’s MULTIMAX Lite data sheet calls for back-buttering tiles 8x8 inches or larger.

Why mixing to spec matters

Thinset is engineered to work at the mix printed on the bag. Follow the bag: the water amount, the mix time, and a rest (slake) period if the bag calls for one. Don’t add extra water to stretch it, and don’t re-temper mortar that has started to set. Too wet or too old, and it won’t bond the way it should, no matter how well it’s troweled.

Water matters too. If you’re on well water and it’s too acidic, be careful: water quality can affect how the mortar mixes and cures. Check your water, follow the mortar maker’s instructions on mixing water, and if you’re unsure, ask the manufacturer or hire a pro.

The exact numbers depend on the product, so we go by LATICRETE’s current instructions for the mortar we’re using. For the specific water ratio or mix time of a given bag, I DON’T KNOW without reading that bag. Check the bag or the data sheet at laticrete.com.

Watch out: “a little more water.” Soupy thinset spreads easily and bonds poorly. We mix to LATICRETE’s instructions every time. Best way? Do your research and hire a pro.

Why coverage checks matter

You can’t see coverage once the tile is down, so you check while you set:

• Lift a tile. Pull one up every so often during setting and look at the back. You want mortar across the back and the edges, not a few stripes.
• Check the ridges collapsed. If the ridges are still standing, the tile wasn’t pressed and slid into place.
• Tap test. Tap the tile gently. A solid tile sounds solid; a hollow, drummy sound can mean empty space behind it.
• Golf ball test. A simple check for homeowners: bounce a golf ball lightly on the tile and listen. A hollow sound can mean empty space behind it.

Checking takes a few minutes. Tearing out a shower takes a lot longer.

Don’t be a pancake installer

There’s a kind of installer I call a pancake pro: five blobs of mortar on the back of the tile and a prayer (more on that in how to choose shower tile). Cookie dots and pancakes of thinset leave hollow spots behind the tile, so it cracks and pops loose. We call it the cookie monster install in our large-format tile guide.

It goes faster on install day. It sounds hollow by year two. In a shower, those hollow spots are exactly where water and condensation collect.

Watch out: the cookie monster install. Dots and blobs of thinset leave hollow spots behind the tile. We back-butter and go for full coverage. Best way? Do your research and hire a pro.

Questions to ask an installer

• What is directional troweling? Ask this one first and just listen to the answer.
• Do you back-butter the tile?
• Which direction do you comb the ridges, and do you slide the tile to collapse them?
• Do you pull a tile during setting to check coverage?
• Which thinset do you use, and do you mix it to the maker’s instructions?
• Is the shower flood-tested before tile goes on?

A good installer will answer these without blinking.

Why hiring a pro matters

Coverage, back-buttering, the mix and the checks are all invisible once the grout goes in. You’re trusting the person doing the work. An experienced tile setter does these things on every tile because they’ve seen what happens when someone doesn’t. That’s why every Burlington Tile job comes with a 25-year warranty on tile-setting workmanship.

Frequently asked questions

What is full thinset coverage?

Mortar contact across the whole back of the tile, with no hollow pockets. On this site, showers call for about 95% mortar coverage, with edges and corners supported.

Can a waterproofed shower still go mouldy?

Yes. Hollow spots behind the tile can hold water, the tile loosens, grout and silicone crack, and moisture sits where it shouldn’t. The membrane is only as good as the coverage under it.

How can I tell if my tile has hollow spots?

Tap the tile gently. A hollow, drummy sound can mean poor coverage behind it. Loose tile and cracked grout are other signs.

Why does the mortar mix matter?

Thinset is engineered to work at the mix the maker prints on the bag. Too much water, the wrong mix time or re-tempering it weakens the bond. We follow LATICRETE’s instructions every time.

Coverage is one layer of a full bathroom remodel. Here’s how we handle the rest.

By Mike Bourdages, Burlington Tile.